Fix up PCbuild/readme.txt a bit.

Removes an incorrect assertion (that the build script will choose the
latest OpenSSL sources it can find), fixes NASM's homepage, and
rewords the section about NASM to make it more clear that you don't
have to install your own.

PCbuild/readme.txt

@@ -138,15 +138,12 @@ _ssl
     obtaining external sources then you don't need to manually get the source
     above via subversion. **
 
-    Alternatively, get the latest version from http://www.openssl.org.
-    You can (theoretically) use any version of OpenSSL you like - the
-    build process will automatically select the latest version.
-
-    You must install the NASM assembler from
-        http://nasm.sf.net
-    for x86 builds.  Put nasm.exe anywhere in your PATH.  If you use the
+    The NASM assembler is required to build OpenSSL.  If you use the
     Tools\buildbot\external(-amd64).bat method for getting sources, it also
     downloads a version of NASM, which the ssl build script will add to PATH.
+    Otherwise, you can download the NASM installer from
+        http://www.nasm.us/
+    and add NASM to your PATH.
 
     You can also install ActivePerl from
         http://www.activestate.com/activeperl/