Fix message of ExceptionForMatrix::ErrOperationNotDefined

```
 % RBENV_VERSION=2.6.5 ruby -r matrix -e 'Vector[1]*Vector[2] rescue p($!)'
 #<ExceptionForMatrix::ErrOperationNotDefined: Operation(*) can't be defined: Vector op Vector>
 % RBENV_VERSION=2.7.0 ruby -r matrix -e 'Vector[1]*Vector[2] rescue p($!)'
 #<ExceptionForMatrix::ErrOperationNotDefined: Operation(*) can\'t be defined: Vector op Vector>
 % RBENV_VERSION=master ruby -r matrix -e 'Vector[1]*Vector[2] rescue p($!)'
 #<ExceptionForMatrix::ErrOperationNotDefined: Operation(*) can't be defined: Vector op Vector>
```

lib/matrix.rb

@@ -38,7 +38,7 @@ module ExceptionForMatrix # :nodoc:
   class ErrOperationNotDefined < StandardError
     def initialize(vals)
       if vals.is_a?(Array)
-        super("Operation(#{vals[0]}) can\\'t be defined: #{vals[1]} op #{vals[2]}")
+        super("Operation(#{vals[0]}) can't be defined: #{vals[1]} op #{vals[2]}")
       else
         super(vals)
       end