Documentation change - Remove MIPS from LIBYUV_DISABLE_DSPR2

Make environment variables match ifdefs with one SIMD instruction set word
as suffix for functions, ifdefs and environment variables.

R=kjellander@chromium.org
BUG=libyuv:486

Review URL: https://codereview.chromium.org/1704053002 .

docs/environment_variables.md

@@ -17,7 +17,7 @@ By default the cpu is detected and the most advanced form of SIMD is used.  But
     LIBYUV_DISABLE_AVX3
     LIBYUV_DISABLE_ERMS
     LIBYUV_DISABLE_FMA3
-    LIBYUV_DISABLE_MIPS_DSPR2
+    LIBYUV_DISABLE_DSPR2
     LIBYUV_DISABLE_NEON
 
 # Test Width/Height/Repeat