Remove POSIX char classes from Awk

I forgot that Debian's awk(1) is still a mawk that doesn't implement
e.g. [:alpha:]

1 files changed, 1 insertions, 1 deletions

diff --git a/bin/mw.awk b/bin/mw.awk
index 48f45fb1..95d70c32 100644
--- a/bin/mw.awk
+++ b/bin/mw.awk
@@ -5,6 +5,6 @@ BEGIN {
 }
 {
     for (i = 1; i <= NF; i++)
-        if ($i ~ /[[:alpha:]]/)
+        if ($i ~ /[a-zA-Z]/)
             print $i
 }