Fix typos throughout codebase.

* NEWS:
* README:
* doc/r5rs/r5rs.texi:
* doc/ref/api-data.texi:
* doc/ref/api-debug.texi:
* doc/ref/api-evaluation.texi:
* doc/ref/api-io.texi:
* doc/ref/api-macros.texi:
* doc/ref/api-procedures.texi:
* doc/ref/api-scheduling.texi:
* doc/ref/api-undocumented.texi:
* doc/ref/libguile-concepts.texi:
* doc/ref/posix.texi:
* doc/ref/srfi-modules.texi:
* doc/ref/vm.texi:
* doc/ref/web.texi:
* examples/box-dynamic-module/box.c:
* examples/box-dynamic/box.c:
* examples/box-module/box.c:
* examples/box/box.c:
* examples/safe/safe:
* examples/scripts/README:
* examples/scripts/hello:
* gc-benchmarks/larceny/twobit-input-long.sch:
* gc-benchmarks/larceny/twobit-smaller.sch:
* gc-benchmarks/larceny/twobit.sch:
* libguile/expand.c:
* libguile/load.c:
* libguile/net_db.c:
* libguile/scmsigs.c:
* libguile/srfi-14.c:
* libguile/threads.c:
* meta/guile.m4:
* module/ice-9/match.upstream.scm:
* module/ice-9/ports.scm:
* module/language/cps/graphs.scm:
* module/scripts/doc-snarf.scm:
* module/srfi/srfi-19.scm:
* module/system/repl/command.scm:
* test-suite/tests/srfi-18.test:
Fix typos.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

README

@@ -172,7 +172,7 @@ switches specific to Guile you may find useful in some circumstances.
   Build a Guile executable and library that supports multi-threading.
 
   The default is to enable threading support when your operating
-  system offsers 'POSIX threads'.  When you do not want threading, use
+  system offers 'POSIX threads'.  When you do not want threading, use
   `--without-threads'.
 
 --enable-deprecated=LEVEL