Fixed the description of init_existential_value to match the implementation

Author: gribozavr

docs/SIL.rst

@@ -4566,11 +4566,11 @@ init_existential_value
 ``````````````````````
 ::
 
-  sil-instruction ::= 'init_existential_value' sil-operand ':' sil-type ','
-                                             sil-type
+  sil-instruction ::= 'init_existential_value' sil-operand ',' sil-type ','
+                                               sil-type
 
-  %1 = init_existential_value %0 : $L' : $C, $P
-  // %0 must be of loadable type $L', lowered from AST type $L, conforming to
+  %1 = init_existential_value %0 : $L, $C, $P
+  // %0 must be of loadable type $L, lowered from AST type $C, conforming to
   //    protocol(s) $P
   // %1 will be of type $P