rpm: corrected dist for CentOS 7.4 to match established naming.

rpm/SPECS/nginx-module.spec.in

diff --git a/rpm/SPECS/nginx-module.spec.in b/rpm/SPECS/nginx-module.spec.in
index 7863be2..028b179 100644
--- a/rpm/SPECS/nginx-module.spec.in
+++ b/rpm/SPECS/nginx-module.spec.in
@@ -17,7 +17,7 @@
 Epoch: %{epoch}
 %define openssl_version %(rpmquery openssl | cut -c 9-13)
 %if "%{openssl_version}" == "1.0.2"
-%define dist .el7.4
+%define dist .el7_4
 %else
 %define dist .el7
 %endif

rpm/SPECS/nginx-plus-module.spec.in

diff --git a/rpm/SPECS/nginx-plus-module.spec.in b/rpm/SPECS/nginx-plus-module.spec.in
index 963d1da..cbce094 100644
--- a/rpm/SPECS/nginx-plus-module.spec.in
+++ b/rpm/SPECS/nginx-plus-module.spec.in
@@ -15,7 +15,7 @@
 %if 0%{?rhel} == 7
 %define openssl_version %(rpmquery openssl | cut -c 9-13)
 %if "%{openssl_version}" == "1.0.2"
-%define dist .el7.4
+%define dist .el7_4
 %else
 %define dist .el7
 %endif

rpm/SPECS/nginx.spec.in

diff --git a/rpm/SPECS/nginx.spec.in b/rpm/SPECS/nginx.spec.in
index 6bd7c05..422735e 100644
--- a/rpm/SPECS/nginx.spec.in
+++ b/rpm/SPECS/nginx.spec.in
@@ -36,7 +36,7 @@
 %if "%{openssl_version}" == "1.0.2"
 Requires: openssl >= 1.0.2
 BuildRequires: openssl-devel >= 1.0.2
-%define dist .el7.4
+%define dist .el7_4
 %else
 Requires: openssl >= 1.0.1
 BuildRequires: openssl-devel >= 1.0.1