Fix single-quotes

Author: isuruf

lib/symengine.rb

@@ -41,9 +41,9 @@ def evalf(operand, prec = 53, real = false)
     def lambdify(exp, *syms)
       syms.flatten!
       if exp.free_symbols.count > syms.length
-        raise ArgumentError, 'Formula contains #{exp.free_symbols.count} free '\
+        raise ArgumentError, "Formula contains #{exp.free_symbols.count} free "\
                              'symbols. You should provide at least this number '\
-                             'of arguments (only #{syms.length} given).'
+                             "of arguments (only #{syms.length} given)."
       end
       # TODO: Implement one of the two options below.
       # 1. check that all the symbol names and function names are valid.