Fix spelling, NFC.

llvm-svn: 241392
diff --git a/llvm/lib/MC/MCAssembler.cpp b/llvm/lib/MC/MCAssembler.cpp
index da6516a..f53b589 100644
--- a/llvm/lib/MC/MCAssembler.cpp
+++ b/llvm/lib/MC/MCAssembler.cpp
@@ -925,7 +925,7 @@
         Fixups = FragWithFixups->getFixups();
         Contents = FragWithFixups->getContents();
       } else
-        llvm_unreachable("Unknow fragment with fixups!");
+        llvm_unreachable("Unknown fragment with fixups!");
       for (const MCFixup &Fixup : Fixups) {
         uint64_t FixedValue;
         bool IsPCRel;