README.md: make the heterogeneous warning more clear

Inspired by merge mixups on the v4.0.x and v4.1.x branches, and also
because the "THIS FUNCTIONALITY..." wording is somewhat ambiguous in
that it's not 100% clear if it applies to the section above or below
the warning, slightly change the wording on the "do not use!" warning
about --enable-heterogeneous to ensure that the warning definitely
applies to the heterogeneous functionality.

Signed-off-by: Jeff Squyres <[email protected]>

Author: jsquyres

README.md

@@ -1506,7 +1506,7 @@ Additionally, if a search directory is specified in the form
   with different endian representations).  Heterogeneous support is
   disabled by default because it imposes a minor performance penalty.
 
-  ***THIS FUNCTIONALITY IS CURRENTLY BROKEN - DO NOT USE***
+  ***THE HETEROGENEOUS FUNCTIONALITY IS CURRENTLY BROKEN - DO NOT USE***
 
 * `--with-wrapper-cflags=CFLAGS`
 * `--with-wrapper-cxxflags=CXXFLAGS`