Fix a typo in the GL_NUM_REQUESTABLE_EXTENSIONS_ANGLE enum value.

GL_NUM_REQUESTABLE_EXTENSIONS_ANGLE had a typo between the spec and the
header but any user who used our header would still get the correct
beaviour.

BUG=angleproject:3861

Change-Id: Ibc85607ad41a8419246e057065ab4ce797f0ce66
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1772376
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>

include/GLES2/gl2ext_angle.h

@@ -20,7 +20,7 @@
 #ifndef GL_ANGLE_request_extension
 #define GL_ANGLE_request_extension 1
 #define GL_REQUESTABLE_EXTENSIONS_ANGLE   0x93A8
-#define GL_NUM_REQUESTABLE_EXTENSIONS_ANGLE 0x93A8
+#define GL_NUM_REQUESTABLE_EXTENSIONS_ANGLE 0x93A9
 typedef void (GL_APIENTRYP PFNGLREQUESTEXTENSIONANGLEPROC) (const GLchar *name);
 #ifdef GL_GLEXT_PROTOTYPES
 GL_APICALL void GL_APIENTRY glRequestExtensionANGLE (const GLchar *name);