Fix mail-extr regexp typo with ".".

Problem reported by Mattias Engdegård in:
https://lists.gnu.org/r/emacs-devel/2019-04/msg00543.html
* lisp/mail/mail-extr.el (mail-extr-telephone-extension-pattern):
Escape the trailing optional period after an abbreviation.

Author: eggert

lisp/mail/mail-extr.el

@@ -383,7 +383,7 @@ by translating things like \"foo!bar!baz@host\" into \"[email protected]\"."
 ;; Matches telephone extensions.
 (defconst mail-extr-telephone-extension-pattern
   (purecopy
-   "\\(\\([Ee]xt\\|[Tt]ph\\|[Tt]el\\|[Xx]\\).?\\)? *\\+?[0-9][- 0-9]+"))
+   "\\(\\([Ee]xt\\|[Tt]ph\\|[Tt]el\\|[Xx]\\)\\.?\\)? *\\+?[0-9][- 0-9]+"))
 
 ;; Matches ham radio call signs.
 ;; Help from: Mat Maessen N2NJZ <[email protected]>, Mark Feit