docs: Add GLSL backend to arch.md

Change-Id: Ifef0b3bb449de2bffc5bc3b84886d4f678971620
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/80521
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Stephen White <senorblanco@chromium.org>
Commit-Queue: Ben Clayton <bclayton@google.com>
diff --git a/docs/arch.md b/docs/arch.md
index 84ec568..204ff64 100644
--- a/docs/arch.md
+++ b/docs/arch.md
@@ -40,17 +40,17 @@
 ┃ Transform ┃◄━━━━━━━━━━━━━━━━━━━━━━━━╋━━━━━━━━━━━━►┃ Inspector ┃
 ┗━━━━━━━━━━━┛                         ┃             ┗━━━━━━━━━━━┛

-┏━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┻━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┓
-┃                                  Writer                                  ┃
-┃                                                                          ┃
-┃ ┏━━━━━━━━━━━━━━┓    ┏━━━━━━━━━━━━━┓    ┏━━━━━━━━━━━━━┓    ┏━━━━━━━━━━━━┓ ┃
-┃ ┃ SPIRV-Writer ┃    ┃ WGSL-Writer ┃    ┃ HLSL-Writer ┃    ┃ MSL-Writer ┃ ┃
-┃ ┗━━━━━━━┳━━━━━━┛    ┗━━━━━━┳━━━━━━┛    ┗━━━━━━┳━━━━━━┛    ┗━━━━━━┳━━━━━┛ ┃
-┗━━━━━━━━━┃━━━━━━━━━━━━━━━━━━┃━━━━━━━━━━━━━━━━━━┃━━━━━━━━━━━━━━━━━━┃━━━━━━━┛
-          ▼                  ▼                  ▼                  ▼
-     ┏━━━━┻━━━┓          ┏━━━┻━━┓           ┏━━━┻━━┓            ┏━━┻━━┓
-     ┃ SPIR-V ┃          ┃ WGSL ┃           ┃ HLSL ┃            ┃ MSL ┃
-     ┗━━━━━━━━┛          ┗━━━━━━┛           ┗━━━━━━┛            ┗━━━━━┛
+┏━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┻━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┓
+┃                                  Writers                                    ┃
+┃                                                                             ┃
+┃ ┏━━━━━━━━━━━━━━┓┏━━━━━━━━━━━━━┓┏━━━━━━━━━━━━━┓┏━━━━━━━━━━━━━┓┏━━━━━━━━━━━━┓ ┃
+┃ ┃ SPIRV-Writer ┃┃ WGSL-Writer ┃┃ HLSL-Writer ┃┃ GLSL-Writer ┃┃ MSL-Writer ┃ ┃
+┃ ┗━━━━━━━┳━━━━━━┛┗━━━━━━┳━━━━━━┛┗━━━━━━┳━━━━━━┛┗━━━━━━┳━━━━━━┛┗━━━━━━┳━━━━━┛ ┃
+┗━━━━━━━━━┃━━━━━━━━━━━━━━┃━━━━━━━━━━━━━━┃━━━━━━━━━━━━━━┃━━━━━━━━━━━━━━┃━━━━━━━┛
+          ▼              ▼              ▼              ▼              ▼
+     ┏━━━━┻━━━┓      ┏━━━┻━━┓       ┏━━━┻━━┓       ┏━━━┻━━┓        ┏━━┻━━┓
+     ┃ SPIR-V ┃      ┃ WGSL ┃       ┃ HLSL ┃       ┃ GLSL ┃        ┃ MSL ┃
+     ┗━━━━━━━━┛      ┗━━━━━━┛       ┗━━━━━━┛       ┗━━━━━━┛        ┗━━━━━┛
 ```
 
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