mgt: Typo in pthread.h (un)safety net

I was trying to add a safety net to catch pthread usage in MGT code and
eventually ran into this. Unfortunately, it didn't catch the pthread.h
inclusion from #4231, because it happened in a header file included
after mgt.h and in general we should avoid including anything from
header files, letting C files include everything needed.

This is at least the model for functions like getaddrinfo(): netdb.h
does not include sys/types.h and sys/socket.h on our behalf.

I'm not sure how to formalize this check beyond a rule to remember
during code reviews. I said in #4346 that I'd like to add a build-time
check but it already exists and for now the only way to uphold it is
discipline.

Author: dridi

bin/varnishd/mgt/mgt.h

@@ -248,7 +248,7 @@ extern const char *mgt_vcl_path;
 extern const char *mgt_vmod_path;
 
 #if defined(PTHREAD_CANCELED) || defined(PTHREAD_MUTEX_DEFAULT)
-#error "Keep pthreads out of in manager process"
+#error "Keep pthreads out of the manager process"
 #endif
 
 #define MGT_FEATURE(x)		COM_FEATURE(mgt_param.feature_bits, x)