Backport speling fix

This change was originall only done for 'master'. Xdebug bundles this file as
it requires it, but PHP 7.2/7.3 don't have it bundled. By doing "phpize" with
master, it updates the file introducing a local diff. By fixing it in older
versions, that problem goes away.

Author: derickr

build/pkg.m4

@@ -86,7 +86,7 @@ dnl Check to see whether a particular set of modules exists. Similar to
 dnl PKG_CHECK_MODULES(), but does not set variables or print errors.
 dnl
 dnl Please remember that m4 expands AC_REQUIRE([PKG_PROG_PKG_CONFIG])
-dnl only at the first occurence in configure.ac, so if the first place
+dnl only at the first occurrence in configure.ac, so if the first place
 dnl it's called might be skipped (such as if it is within an "if", you
 dnl have to call PKG_CHECK_EXISTS manually
 AC_DEFUN([PKG_CHECK_EXISTS],