Adjust docs example to work with v0.8.0

docs/contracts/libraries.rst

@@ -47,7 +47,7 @@ more advanced example to implement a set).
 ::
 
     // SPDX-License-Identifier: GPL-3.0
-    pragma solidity >=0.6.0 <0.8.0;
+    pragma solidity >=0.6.0 <0.9.0;
 
 
     // We define a new struct datatype that will be used to
@@ -126,7 +126,7 @@ custom types without the overhead of external function calls:
 ::
 
     // SPDX-License-Identifier: GPL-3.0
-    pragma solidity >=0.6.0 <0.8.0;
+    pragma solidity >=0.6.0 <0.9.0;
 
     struct bigint {
         uint[] limbs;
@@ -243,7 +243,7 @@ Its value can be obtained from Solidity using the ``.selector`` member as follow
 ::
 
     // SPDX-License-Identifier: GPL-3.0
-    pragma solidity >=0.5.14 <0.8.0;
+    pragma solidity >=0.5.14 <0.9.0;
 
     library L {
         function f(uint256) external {}