net/addr/ip: Use cheaper char find

Incorrectly using string find instead of char find

Change-Id: I436801171de05efde6b118a46f8076d569c31a77
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/include/stdplus/net/addr/ip.hpp b/include/stdplus/net/addr/ip.hpp
index fdef5c3..d5e5312 100644
--- a/include/stdplus/net/addr/ip.hpp
+++ b/include/stdplus/net/addr/ip.hpp
@@ -121,7 +121,7 @@
         uint32_t addr = {};
         for (size_t i = 0; i < 3; ++i)
         {
-            auto loc = sv.find(".");
+            auto loc = sv.find('.');
             addr |= sti(sv.substr(0, loc));
             addr <<= 8;
             sv.remove_prefix(loc == sv.npos ? sv.size() : loc + 1);