[module.unit] Add "either" to clarify that we're talking about

module-names containing a reserved identifier, not module names starting
with an identifier that contains a reserved identifier.

Author: zygoloid

source/modules.tex

@@ -35,7 +35,7 @@
 shall not appear as \grammarterm{identifier}{s}
 in a \grammarterm{module-name} or \grammarterm{module-partition}.
 \indextext{module!reserved name of}%
-All \grammarterm{module-name}{s} beginning with an \grammarterm{identifier}
+All \grammarterm{module-name}{s} either beginning with an \grammarterm{identifier}
 consisting of \tcode{std} followed by zero or more \grammarterm{digit}{s} or
 containing a reserved identifier\iref{lex.name}
 are reserved and shall not be specified in a \grammarterm{module-declaration};