[lex.string] Replace colon with full stop to end the sentence.

On request of ISO/CS, for otherwise we should have made the next 'if'
lowercase, because: "Grammatically this is the same sentence,
as there is no full stop, so the 'if' should be lowercase."

If a colon really doesn't end a sentence, then we should make it so
that the sentence really does end.

Author: tkoeppe

source/lex.tex

@@ -1901,7 +1901,7 @@
 \indextext{concatenation!string}%
 The common \grammarterm{encoding-prefix}
 for a sequence of adjacent \grammarterm{string-literal}s
-is determined pairwise as follows:
+is determined pairwise as follows.
 If two \grammarterm{string-literal}{s} have
 the same \grammarterm{encoding-prefix},
 the common \grammarterm{encoding-prefix} is that \grammarterm{encoding-prefix}.