patch 8.2.0156: various typos in source files and tests

Problem:    Various typos in source files and tests.
Solution:   Fix the typos. (Emir Sari, closes #5532)
diff --git a/src/testdir/test_digraph.vim b/src/testdir/test_digraph.vim
index 6435c86..e66663b 100644
--- a/src/testdir/test_digraph.vim
+++ b/src/testdir/test_digraph.vim
@@ -80,7 +80,7 @@
   call Put_Dig(".e")
   call Put_Dig("a.") " not defined
   call assert_equal(['ḃ', 'ė', '.'], getline(line('.')-2,line('.')))
-  " Diaresis
+  " Diaeresis
   call Put_Dig("a:")
   call Put_Dig(":u")
   call Put_Dig("b:") " not defined
@@ -285,7 +285,7 @@
   call Put_Dig_BS(".","e")
   call Put_Dig_BS("a",".") " not defined
   call assert_equal(['ḃ', 'ė', '.'], getline(line('.')-2,line('.')))
-  " Diaresis
+  " Diaeresis
   call Put_Dig_BS("a",":")
   call Put_Dig_BS(":","u")
   call Put_Dig_BS("b",":") " not defined