PR #1720: Fix spelling mistake: occurrance -> occurrence

Imported from GitHub PR https://github.com/abseil/abseil-cpp/pull/1720

Merge a72c4bcc7b45d17cf12bee62989722f16f85f0fd into 0598e582

Merging this change closes #1720

COPYBARA_INTEGRATE_REVIEW=https://github.com/abseil/abseil-cpp/pull/1720 from eltociear:patch-1 a72c4bcc7b45d17cf12bee62989722f16f85f0fd
PiperOrigin-RevId: 653737423
Change-Id: I5638538d1b008942feba6dc6fc98dcdbf53653ef

absl/strings/cord.h

@@ -772,7 +772,7 @@ class Cord {
 
   // Cord::Find()
   //
-  // Returns an iterator to the first occurrance of the substring `needle`.
+  // Returns an iterator to the first occurrence of the substring `needle`.
   //
   // If the substring `needle` does not occur, `Cord::char_end()` is returned.
   CharIterator Find(absl::string_view needle) const;