README: fix example code

[olshevskiy87]

• add "header" sections (package and import)
• add missing word "struct" in the User type declaration
• replace "new" with composite literal to get rid of unnecessary assignments
• add missing returned error receiver to MCache.Set and MCache.SetPointer
• fix MCache name where methods Get and GetPointer are called
• fix typo objUser,Bio -> objUser.Bio
• last log.Println should be Printf in order to replace "%s" with the key value

[OrlovEvgeny]

Thank