BUILDING typos

[Tobias Ulmer <tobiasu%tmux.org@localhost>]

Obvious fix

Index: BUILDING
===================================================================
RCS file: /usr/cvsroot/src/BUILDING,v
retrieving revision 1.134
diff -u -p -r1.134 BUILDING
--- BUILDING	29 Jul 2018 20:53:41 -0000	1.134
+++ BUILDING	8 May 2019 13:04:48 -0000
@@ -206,7 +206,7 @@ CONFIGURATION
                  Directory containing sources of externally added programs and
                  libraries.  If specified, must be an absolute path.
 
-                 Default: NETBSDRCDIR/../extsrc, if that exists; otherwise
+                 Default: NETBSDSRCDIR/../extsrc, if that exists; otherwise
                  /usr/extsrc.
 
      MAKECONF    The name of the make(1) configuration file.  Only settable in
@@ -461,7 +461,7 @@ CONFIGURATION
                  must be an absolute path.  The main modular Xorg source is
                  found in X11SRCDIR/external/mit.
 
-                 Default: NETBSDRCDIR/../xsrc, if that exists; otherwise
+                 Default: NETBSDSRCDIR/../xsrc, if that exists; otherwise
                  /usr/xsrc.
 
    "make" variables for full builds
Index: doc/BUILDING.mdoc
===================================================================
RCS file: /usr/cvsroot/src/doc/BUILDING.mdoc,v
retrieving revision 1.125
diff -u -p -r1.125 BUILDING.mdoc
--- doc/BUILDING.mdoc	29 Jul 2018 20:53:29 -0000	1.125
+++ doc/BUILDING.mdoc	8 May 2019 13:05:14 -0000
@@ -430,7 +430,7 @@ Directory containing sources of external
 and libraries.
 If specified, must be an absolute path.
 .DFLT
-.Sy NETBSDRCDIR Ns Pa /../extsrc ,
+.Sy NETBSDSRCDIR Ns Pa /../extsrc ,
 if that exists; otherwise
 .Pa /usr/extsrc .
 .
@@ -814,7 +814,7 @@ If specified, must be an absolute path.
 The main modular Xorg source is found in
 .Sy X11SRCDIR Ns Pa /external/mit .
 .DFLT
-.Sy NETBSDRCDIR Ns Pa /../xsrc ,
+.Sy NETBSDSRCDIR Ns Pa /../xsrc ,
 if that exists; otherwise
 .Pa /usr/xsrc .
 .