Fix typos in comment (#796)

* s/Calcualte/Calculate/

* s/unneccesary/unnecessary/

Author: ydah

lib/reline/line_editor.rb

@@ -1188,7 +1188,7 @@ def retrieve_completion_block
     quote_characters = Reline.completer_quote_characters
     before = current_line.byteslice(0, @byte_pointer).grapheme_clusters
     quote = nil
-    # Calcualte closing quote when cursor is at the end of the line
+    # Calculate closing quote when cursor is at the end of the line
     if current_line.bytesize == @byte_pointer && !quote_characters.empty?
       escaped = false
       before.each do |c|

lib/reline/unicode.rb

@@ -61,7 +61,7 @@ def self.safe_encode(str, encoding)
 
     # This code is essentially doing the same thing as
     # `str.encode(utf8, **replace_options).encode(encoding, **replace_options)`
-    # but also avoids unneccesary irreversible encoding conversion.
+    # but also avoids unnecessary irreversible encoding conversion.
     converted.gsub(/\X/) do |c|
       c.encode(Encoding::UTF_8)
       c